1. Field of the Invention
This invention is in the field of powder metallurgy and involves a combination of steps including pressing a powder mixture, sintering the resulting preform to cause alloying and solid state diffusion and then forming the preform into the required shape at substantially theoretical density by means of backward extrusion. This extrusion may and preferably is followed by a carburizing treatment and a series of reheating steps with intermediate quenching to achieve the hardness desired in the finished product.
2. Description of the Prior Art
Valve lifters used today are usually composed of cast iron, cast iron alloys, composites, or other expensive ferroalloy materials. Significant improvements in the art of forming high density parts by means of cold extrusion of ferrous metal powder are described in our previous U.S. Pat. No. 3,060,560. The disclosure of that patent in its entirety is incorporated herein by reference.
U.S. Pat. No. 3,150,444 describes a method of producing a heat hardenable steel for use in high speed tools which involves compressing atomized pre-alloyed powder of the steel, sintering the compacted powder in the presence of a reducing atmosphere, and mechanically working the compacted and sintered powder so as to achieve a density approaching the theoretical density of the metal.
U.S. Pat. No. 3,198,182 is directed specifically to the manufacture of valve lifters. In this patent, there is described a procedure wherein an intimate mixture of powdered carbon, tungsten, molybdenum, silicon, and iron is compressed to form a thin briquette. This briquette is placed in a shallow cavity formed in the working end of a valve lifter and the briquette and adjacent valve lifter surfaces are joined together by a heating at a temperature of about 2200.degree. F. thereby creating a diffusion bond with the remainder of the valve lifter. This patent is typical of attempts to provide bimetallic surfaces of a harder composition on the working end of a valve lifter.
In U.S. Pat. No. 3,200,801 there is described a valve lifter having a two-piece body in which the major portion is formed of a stainless steel tubular member, and a minor portion is formed of a low alloy steel tubular member. The low alloy steel portion is interposed between the stainless steel portion and an alloy cast iron foot piece of the valve lifter.
U.S. Pat. No. 3,244,506 describes a powder metallurgy method utilizing a pre-alloyed atomized metal powder wherein the powder is formed into a metal cutting tool and the metal is reacted to form finely dispersed carbides in a fine grain metal matrix.
Robinson et al U.S. Pat. No. 3,255,513 is similar to the aforementioned U.S. Pat. No. 3,198,182 but is directed specifically to using a briquette composed of a powdered metal mixture of substantial amounts of carbon, molybdenum, tungsten, silicon, and iron.
In U.S. Pat. No. 3,655,365 there are described compositions for use as tools, formed by the hot consolidation of pre-alloyed powders having a uniformly dispersed carbide phase of a grain size less than 3 microns. The alloy contains from 10 to 40% of tungsten or molybdenum, from 0.5 to 4% carbon, a carbide former, and a mixture of iron and cobalt as the balance.
In U.S. Pat. No. 3,657,800 there is described a valve lifter employing a wear plate of graphitic alloy steel which is friction welded to a steel tube in an attempt to improve the scuff resistance at the point of contact with the camshaft.
A hydraulic tappet having a barrel formed of powdered metal which is sintered and compacted, and employing a cam face of a sintered component infiltrated with a hardening agent is described in U.S. Pat. No. 3,683,876. Optionally, the cam face may be a separate sintered metal disc suitably attached to the tappet barrel.
In U.S. Pat. No. 3,690,959 there is described a tappet made of a high carbon, high chromium alloy. The alloy is cast, cooled quickly to form a relatively small number of relatively large primary chromium carbide particles dispersed in a matrix of austenite containing a solid solution of chromium and carbon. Large numbers of relatively small particles of chromium carbides are then precipitated on the matrix and distributed throughout the spaces between the large primary carbon particles. The casting is hardened by heating and subsequent quenching to convert the matrix to martensite without changing the carbide particles.
The problem of reducing oxide inclusions which is important in this technology is referred to in U.S. Pat. No. 3,740,215. That patent describes a method of consolidating powder composed of metallic particles into a porous body, substantially sealing the surface of the body by closing the surface pores, heating the body, and then hot working it.
U.S. Pat. No. 3,832,763 refers to the problem of densifying sintered workpieces by drop-forging them in a die resulting in a massive deformation in the drop-forging step.
U.S. Pat. No. 3,867,751 is directed specifically to the manufacture of inner and outer bearing rings but also addresses itself to the problem of densifying sintered powdered metal blanks. In this patent, there is disclosed a method wherein the sintered powdered metal blank having a density of at least 96% is roll formed to the shape of the inner or outer bearing ring.
Powdered metal parts having bearing surfaces are described in U.S. Pat. No. 3,874,049. The method involved in that patent consists in cold forming a sintered preform through the application of shear forces to the surface of the preform where the bearing surface is desired, by causing a movable die to penetrate and wipe along such surface of the preform.
Powder metallurgy forging is described in U.S. Pat. No. 3,897,618. In this patent, steel powder is forged at a temperature at which the steel is characterized by a microstructure containing specified percentages of ferrite and austenite.
In U.S. Pat. No. 3,992,763 there is described a method of making powdered metal parts wherein a briquetted powder metal preform is carburized by means of a gas atmosphere and then hot forged to produce a highly densified, carburized powdered metal part.
In U.S. Pat. No. 4,051,590 there is described an automated method of hot forging articles from powder metal preforms. The preforms are passed through an induction heating device in which they are heated to a forging temperature. The heated preforms are then forged into finished articles, after which they are cooled.
Surface densification of a powdered metal part is the subject of U.S. Pat. No. 4,059,879. The method involved in this patent applies densifying pressure to a selected portion of a sintered powdered metal element while applying restraining pressures to other portions of the element in order to inhibit growth and cracking of the element during the cold deformation which occurs.
Finally, U.S. Pat. No. 4,086,087 describes a method for producing powdered metal parts wherein shaped powdered metal preforms are treated with an impregnant which is immiscible with organic lubricants, the metal part is treated with a lubricant, an then sized or coined. The sized powdered metal part is preferably also heat treated.